Power amplifiers are frequently used as switches to drive various kinds of electrical loads, such as for example, motors, power supplies, CRT deflection yokes, transformers, inductors, capacitors, resistors and combinations thereof. It is commonplace for such power amplifiers to switch currents of from one to hundreds of amperes or more and to stand off voltages of from a hundred to several thousand volts or more. The exact combination of forward conduction current and blocking voltage that must be handled by the power amplifier will depend upon the energy source, the load and the desired waveform. As is well known in the art, a variety of other operating parameters of the energy source, the load and the switch are important for design of a reliable and economical system.
All amplifiers dissipate energy internally while operating. At DC or low frequencies, the transient (i.e., switching) energy loss is negligible and the principal energy loss is determined by the product of the forward current times the forward voltage drop when the switch is on. The loss due to leakage current flowing when the amplifier or switch is off is usually negligible.
As the operating frequency of the amplifier increases the transient or switching energy loss increases. The transient or switching energy loss is that energy loss which occurs when the amplifier or switch is changing from the conductive to the non-conductive state and/or vice-versa. Other things being equal, the transient energy loss increases in direct proportion to the operating frequency. For many power amplifiers the transient energy loss can become very significant at frequencies of a few kilohertz or more. This is especially true in transistor amplifiers which have been designed to have low forward voltage drop.
One application where the transient energy loss is of significant concern is in power amplifiers driving inductive or resonant loads, as for example, the yoke of a cathode ray tube (CRT) deflection system. The horizontal deflection amplifier is usually the more difficult to accomplish since it typically operates at a higher frequency than the vertical deflection amplifier. Prior art horizontal deflection amplifiers or systems are described, for example, in U.S. Pat. No. 4,670,692, 4,642,533, 4,205,259, 3,501,672 and 3,480,826, which are incorporated herein by reference.
The switching rate or operating frequency of the deflection amplifier is one factor that determines the degree of resolution of an image formed on the CRT. As the need has increased for progressively higher resolution, so has deflection amplifier operating frequency. Deflection amplifiers operating at 64-270 kHz or higher are now much desired. An improved horizontal deflection amplifier is described in U.S. Pat. No. 4,897,580 to Schultz which is also incorporated herein by reference.
Despite various attempts to reduce the internal energy dissipation in power amplifiers, significant problems well known in the art remain. This is especially true for power amplifiers operating at higher frequencies where transient (i.e., switching) energy losses predominate.
A further problem well known in the art is that active devices used in power amplifiers are subject to considerable variations in parameters from device to device. Thus, while a particular amplifier circuit may be adjusted to provide minimum dissipation with a particular amplifier device or combination of devices, if another nominally identical amplifier device is substituted, the circuit must be readjusted in order to still provide minimum dissipation. Such tuning or tweaking of the circuit to match the properties of individual devices is impractical in systems which must be manufactured in large volume and at low cost. The problem is further complicated by the fact that the various properties of active devices typically used in such power amplifiers are interrelated and optimization of one parameter, e.g., forward voltage drop may adversely affect other parameters, e.g., turn-off time, or vice versa.